Don't buy any more? Force yourself to stay inside. It may be my lazy/whatever personality, but for me, at least, staying home is easy, no matter what happens.
If you already did, flush it into the toilet. Oops.
If you want to prevent yourself from buying any more, stop carrying cash altogether. In a modern world, all your expenses can be easily covered with a card, in 99 % stores. I doubt pushers take Mastercard.
Oh, yeah, and crystal meth is something that you take by the nose, right? Then throw away any material you might have for it. Anything I know is from movies, but, uhm... Don't keep any straws or stuff.
If you really think you can't resist it, go to the hospital. They're used to such situations, and they'll keep you from any dangerous substance until the physical urge is gone, at least. They can probably refer you to an addiction therapist, too, which are really effective, in general.
If you really can't go to the hospital, how about rendering yourself unable to buy drugs? It's not the healthiest thing on Earth, but if you ask me, taking a sleeping pill or two should be much better than crystal meth. If you don't have sleeping pills, do you have Benadryl or a similar quick-action allergy relief medication? One or two of those make me dizzy and really, really sleepy.
You know, once you buy the drug, you're in a really bad situation. It's human and natural to want to use what you paid for. Put the emphasis on resisting the purchase. Everything is a hundred times harder once you have the powder on hand.